Javascript try...catch...else...finally like Python, Java, Ruby, etc

Question

How can Javascript duplicate the four-part try-catch-else-finally execution model that other languages support?

A clear, brief summary is from the Python 2.5 what's new. In Javascript terms:

// XXX THIS EXAMPLE IS A SYNTAX ERROR try {   // Protected-block } catch(e) {   // Handler-block } else {   // Else-block } finally {   // Final-block } 

The code in Protected-block is executed. If the code throws an exception, Handler-block is executed; If no exception is thrown, Else-block is executed.

No matter what happened previously, Final-block is executed once the code block is complete and any thrown exceptions handled. Even if there’s an error in Handler-block or Else-block and a new exception is raised, the code in Final-block is still run.

Note that cutting Else-block and pasting at the end of Protected-block is wrong. If an error happens in Else-block, it must not be handled by Handler-block.

Answer 1

I know this is old, but here is a pure syntax solution, which I think is the proper way to go:

try {     // Protected-block     try {         // Else-block     } catch (e) {         // Else-handler-block     } } catch(e) {     // Handler-block } finally {     // Final-block } 

The code in Protected-block is executed. If the code throws an error, Handler-block is executed; If no error is thrown, Else-block is executed.

No matter what happened previously, Final-block is executed once the code block is complete and any thrown errors handled. Even if there’s an error in Handler-block or Else-block, the code in Final-block is still run.

If an error is thrown in the Else-block it is not handled by the Handler-block but instead by the Else-handler-block

And if you know that the Else-block will not throw:

try {     // Protected-block     // Else-block } catch(e) {     // Handler-block } finally {     // Final-block } 

Moral of the story, don't be afraid to indent ;)

Note: this works only if the Else-handler-block never throws.

Answer 2

Extending the idea of jhs a little, the whole concept could be put inside a function, to provide even more readability:

var try_catch_else_finally = function(protected_code, handler_code, else_code, finally_code) {   try {     var success = true;     try {       protected_code();     } catch(e) {       success = false;       handler_code({"exception_was": e});     }     if(success) {       else_code();     }   } finally {     finally_code();   } }; 

Then we can use it like this (very similar to the python way):

try_catch_else_finally(function() {   // protected block }, function() {   // handler block }, function() {   // else block }, function() {   // final-block });